1. Field of the Invention
The present invention relates to a front-and-rear wheel drive vehicle in which one of pairs of front and rear wheels is driven by an engine, while the other pair is driven by start assisting motors.
2. Description of the Related Art
A front-and-rear wheel drive vehicle as described above is known in, for example, Japanese Patent Unexamined Publication No. Hei. 8-175209. In addition, a technology is also known in which a low speed-high torque characteristic and a high speed low-torque characteristic are allowed to coexist with each other by a single field-control motor, which can change the field current thereof without provision of a transmission by making the field current of the motor strong when the motor rotation number is slow and the field current weak when the motor rotation number is fast.
When the field-control motor is used as a start assisting motor for a front-and-rear wheel drive vehicle, the start assisting performance can be improved by switching the low speed-high torque characteristic to the high speed-low torque characteristic or vice versa in response to a change in motor rotation number. This, however, requires a motor rotation number sensor for detecting the rotation number of the field-control motor and results in a increase of cost.
The present invention was made in view of the aforesaid problem, and an object thereof is to enable the utilization of the field-control motor described above as a start assisting motor for a front-and-rear wheel drive vehicle without the necessity of a special motor rotation number sensor.
With a view to attaining the aforesaid object, according to a first feature of the invention, there is provided a front-and-rear wheel drive vehicle comprising an engine for driving a pair of front wheels and rear wheels. Also included are motors for driving the other pair of front wheels and rear wheels, wherein the motors drive the other pair of wheels when a vehicle starts to render assistance in starting, and wherein the assistance in starting is stopped when the vehicle speed reaches a predetermined value. The motors comprise field-control motors, and the control means is provided for controlling field current of the motors based on outputs from the speed sensors for detecting the rotating speeds of the other pair of wheels.
According to the above construction, since the speed of the motor and the speed of the wheels rotated by the motor has a certain relationship, there is no need to provide a speed sensor for detecting the rotating speed of the wheels rotated by the motor, and the vehicle""s start assisting performance can be improved by controlling the field current of the motor based on outputs from the speed sensor for detecting the rotating speed of the wheels to switch the low speed-high torque and high speed-low torque characteristics of the motor at will. In particular, utilization of the speed sensor for this purpose, which is initially provided on the vehicle for the anti-lock brake and traction control systems, can contribute to reduction in the number of components involved and production cost.